1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a liquid crystal display device and a driving method thereof that are adaptive for improving a picture quality.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmittance of a liquid crystal using an electric field in order to display a picture. To achieve this, the LCD includes a liquid crystal display panel having a pixel matrix and a driving circuit for driving the liquid crystal display panel. The driving circuit drives the pixel matrix such that picture information may be displayed on the display panel.
Referring to FIG. 1, the related art LCD includes a liquid crystal display panel 2, a data driver 4 for driving data lines DL1 to DLm of the liquid crystal display panel 2, and a gate driver 6 for driving gate lines GL1 to GLn of the liquid crystal display panel 2.
The liquid crystal display panel 2 includes of thin film transistors TFT at each crossing of the gate lines GL1 to GLn and the data lines DL1 to DLm and liquid crystal cells connected to the thin film transistors TFT and arranged in a matrix.
The gate driver 6 sequentially applies a gate signal to each gate line GL1 to GLn in response to a control signal from a timing controller (not illustrated). The data driver 4 converts data R, G and B from the timing controller into analog video signals that are applied one horizontal line at a time to the data lines DL1 to DLm every one horizontal period when a gate signal is applied to each gate line GL1 to GLn.
The thin film transistor TFT applies data from the data lines DL1 to DLm to the liquid crystal cell in response to a control signal from the gate lines GL1 to GLn. The liquid crystal cell may be equivalently expressed as a liquid crystal capacitor Clc because it has a common electrode opposed to a pixel electrode connected to the thin film transistor TFT and having a liquid crystal therebetween. Such a liquid crystal cell includes a storage capacitor (not illustrated) connected to a pre-stage gate line in order to keep the data voltage charged in the liquid crystal capacitor Clc until the next data voltage is charged therein.
Such a related art LCD requires operating waveforms as illustrated in FIG. 2 so as to drive the liquid crystal cell.
FIG. 2 is a waveform diagram of a common electrode voltage Vcom, each gate electrode voltage and each data voltage applied to the related art LCD.
Referring to FIG. 2, a common electrode voltage Vcom is applied and a gate signal for driving the thin film transistor TFT is applied. If the thin film transistor TFT is turned on by such a gate signal, then a positive data voltage Vdata(+) is charged into the liquid crystal cell (at a charging area). Thereafter, if the thin film transistor TFT is turned off, then the data voltage Vdata(+) charged by the storage capacitor is maintained (at a maintaining area).
Next, if a gate signal for driving the thin film transistor TFT is re-applied to the gate line at the next frame, then a negative data voltage Vdata(−). Thereafter, if the thin film transistor TFT is turned off, then the data voltage Vdata(−) charged by the storage capacitor is maintained.
When the thin film transistor TFT is turned on to charge a voltage into the liquid crystal cell (at the charging area) and then the thin film transistor TFT is turned off (at the maintaining area). At the thin film transistor TFT, a liquid crystal voltage is varied by ΔVp by a capacitance between a gate electrode G and a source electrode S.
If a sequential gate signal progressing from the upper portion of the liquid crystal display panel 2 toward the lower portion thereof is inputted to the gate line in this manner, then each thin film transistor TFT is simultaneously turned on by an input of the gate signal and a displaying data voltage is inputted from the data line for each pixel. Thus, the data voltage is applied to a pixel electrode, and transmittance of the liquid crystal is changed by a potential difference between the voltage at the pixel electrode and the common electrode voltage.
However, in the thin film transistor TFT of the LCD, roles of a source electrode S and a drain electrode D when the data voltage has a positive(+) polarity are exchanged from roles of the source electrode S and a drain electrode D when the data voltage has a negative(−) polarity. In other words, any one having a lower potential of a data voltage Vdata applied to the data line and a voltage at the liquid crystal capacitor Clc performs a role of the source electrode.
In FIG. 2, the gate signal turning on the thin film transistor TFT is applied as the same gate signal regardless of the polarity of the data voltage Vdata. Thus, a potential difference Vgs1 between said two voltages when a positive data voltage Vdata(+) is applied at the current frame is differentiated from a potential difference Vgs2 between said two voltages when a negative data voltage Vdata(−) is applied at the next frame. Accordingly, the amount of current flowing in the thin film transistor TFT is differentiated causing an imbalance of electric charge in the liquid crystal cell. As a result, a deterioration in picture quality is caused, such as flickering or a residual image, for example.